As shown in FIG. 13, it is known to provide an automobile 1 with an obstacle detection apparatus which has a case 2 containing an ultrasonic wave transmitter and a receiver disposed at a corner part 3 of the vehicle body for preventing collision at the corner during garaging or the like. An ultrasonic wave 4 is transmitted from the corner part 3 and its reflection wave 5 is received to determine the distance from the corner part 3 to an obstacle 6 based upon the delay time from the transmission to the reception and notify the driver of the distance. An piezoelectric element is used for the transmission and reception of the ultrasonic wave. In this case, since an ultrasonic wave is a compression wave of air, the piezoelectric element receives the compression wave to produce a deformation, and the deformation generates an electromotive force by the piezoelectric effect of the element. When a piezoelectric element is used as an ultrasonic wave reception sensor, since the piezoelectric element itself has a natural frequency, and it is necessary to select a specific resonance frequency region by selecting its material, size, and shape in order to improve the sensitivity, it is difficult to receive ultrasonic wave 5 over a wide frequency region. Furthermore, the transmitter and the receiver have a substantial directional dependency.
The above-described prior art obstacle detection apparatus is effective for preventing collision of the corner part 3. However, as shown in FIG. 14, when an obstacle, for example, another automobile 7, is present at the right or left of the automobile 1, it may sometimes be impossible to receive the reflection wave 5 and detect the other vehicle 7. To prevent such a problem, it has been considered to provide the transmitter and receiver not only at the corner part 3, but as disclosed in Japanese Utility Model Laid-open Publication 63-69654, a number of the devices may be provided over the entire periphery of the vehicle. However, this results in large parts cost and installation cost. Furthermore, even if the piezoelectric element is increased in length to expand the reception zone, the resonance frequency is decreased to the extent, and not resonant with nor receive the ultrasonic wave, which is not practical.
Furthermore, the danger of collision with an obstacle depends not only on the distance but also on the relative velocity with the obstacle. However, with the prior art obstacle detection apparatus, which uses a piezoelectric element, it is difficult to measure the relative velocity. Specifically, even if the Doppler effect is used to measure a relative velocity, the frequency of the reflection wave may be out of the resonance frequency of the reception piezoelectric element, and the reflection wave is not detected.